Apparatus for mooring a vessel to a submerged mooring element

ABSTRACT

A system for mooring oil transport, production, and drilling vessels in sea ice in the Arctic. The mooring system combines a submerged buoyant element structurally connected, for vertical movement, to an anchor structure on the seabed, and is designed to anchor a vessel equipped with a mooring system including a device for evacuating seawater from the mooring area between the hull of the vessel and the mooring element. The system can also be used without a vertically slidable mooring element, so that the vessel is directly fixed to the anchor structure using the device for evacuating seawater.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the mooring of oil transport,production, and drilling vessels in sea ice in the Arctic. Moreparticularly the invention relates to a mooring system which combines asubmerged buoyant element structurally connected to an anchor structureon the seabed, and designed to anchor a vessel equipped with a mooringsystem of the type described in U.S. Pat. Nos. 5,305,703 and 5,477,114.

2. Discussion of the Prior Art

Currently, moorings for vessels in sea ice do not exist. However, anumber of proposals have been advanced, using single point moorings ofthe tower type in which the vessel moors by a structural connection atthe deck level of the vessel to a pivoting structure mounted on top of afixed tower protruding up through the ice. Very large forces need to betransferred in such a device, on the order of 50 to 100 MN. Forces ofthis magnitude exceed by a large factor the breaking strength of thelargest commercially available chain or rope, therefore speciallydesigned very large structural connectors are required.

An alternative means of station keeping that has been proposed is to usehigh powered, dynamically positioned icebreaking vessels assisted bynuclear or conventionally powered ice breakers. This technology may befeasible for oil shuttle tankers that can tolerate being forced off themooring, by discontinuing the oil transfer until it can return to theloading point. Such force-offs are, however, much less acceptable foroil production or drilling vessels because in oil shuttle tankers thecrude oil can be continually produced into a buffer storage while theshuttle tanker is unavailable, whereas forcing a production vesseloff-station causes oil production shut-in, and forcing a drilling vesseloff-station with insufficient warning may have catastrophicconsequences.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved mooringsystem of the single-point mooring type, permitting the rapid and securemooring of ships in ice-infested waters.

Another objective is to provide a mooring capable of transmitting forcesas large as 500 MN between the vessel and the sea bed.

Still another object is to provide a mooring that permits the vessel toweather vane in response to the change of direction of the drifting ice.

The above and other objects are met by providing a submerged mooringelement that is engageable to the mooring area of a vessel equipped withan apparatus to reduce the hydrostatic pressure in the mooring area suchas is described in U.S. Pat. Nos. 5,305,703 and 5,477,114.

An anchor structure on the sea bed in structural contact with thesubmerged mooring element is slidably engaged so that horizontal motionsare resisted but vertical motion and rotation in the horizontal plane ispermitted.

The upper part of the buoyant mooring element preferably includes atleast one resilient annular member concentric with the vertical axis ofthe mooring element, the resilient annular member making initial contactwith the mooring recess to cushion any impact between the mooringelement and the vessel. Preferably the resilient annular member makes acircle of sealing contact with the bottom of the hull so that the devicefor rapidly drawing seawater away from the mooring area can pump out theregion between the bottom of the hull and the upper part of the mooringelement inside the circle of sealing contact. The upper part of themooring element, or the lower part of the hull of the vessel, caninclude two concentric resilient annular members that makes circles ofsealing contact at locations that are respectively radially inside andradially outside the location of the intake of the device for drawingaway seawater, so that the downward pressure on the upper part of themooring element between the concentric circles of sealing contact can bereduced to a level possibly as low as the vapor pressure of theseawater.

It is noted that vessels that are ice bound are not subjected to dynamicforces from wave action, therefore, very little flexibility or energyabsorbing capacity is needed in a mooring. Normally such flexibility isprovided in a mooring by catenary anchor chains, flexible ropes, or acombination thereof such as for example described in U.S. Pat. No.5,305,703. However, ice bound vessels are subjected to extreme forcesfrom the drifting ice. Only if the vessel is moored with a mooringcapable of supplying the force required to break the drifting ice canthe vessel remain moored.

The present invention pertains in particular to the mooring of vesselsequipped to moor to a buoy held by hydrostatic pressure differentialssuch as described in U.S. Pat. Nos. 5,305,703 and 5,477,114. The buoysdescribed in the two referenced U.S. patents are normally circular andin the description below it is assumed that the buoys are circular in atop view. However, it is not required that the buoys be circular andeven larger forces could be obtained from a non-circular buoy. As anexample of the capabilities of this invention, consider the mooring of a250,000 DWT vessel with the following main dimensions: length betweenperpendiculars: 350 m, draft: 22 m, and breadth: 55 m. This vessel canaccommodate a circular buoy 50 m in diameter. Such a buoy has a surfacearea of 2000 m². The fully loaded vessel has an absolute hydrostaticpressure at the keel of 320 kPa. Assume that the pressure above the buoyis lowered to 50 kPa, for example by pumps aboard the vessel withdrawingwater from the volume isolated from the sea by the buoy and the vessel.The attractive force between the buoy and the vessel would be 540 MN.Assuming a friction coefficient of 0.5 between the buoy and the vessel,this results in the buoy being able to transmit a horizontal force of270 MN to the vessel. Typical forces that a vessel must resist in theice are in the range of 30 to 150 MN--however, they can be higher. Inthe event that the mooring force exceeds the capacity of this mooring,the mooring buoy is simply forced off the vessel by sliding along thebottom of the vessel. In contrast vessels that are moored withmechanical links must incorporate release mechanisms that release themooring when the allowable load is exceeded. For the loads in questionsuch devices must be very large.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side view of a first embodiment of the present invention;

FIG. 1a shows a detail view of the buoyancy chamber of the embodiment ofFIG. 1;

FIG. 2 shows a side view of a modification of the first embodiment ofthe present invention;

FIG. 3 shows a side view of a second embodiment of the presentinvention;

FIG. 4 shows a top view of a third embodiment of the present invention;

FIG. 5 shows a side view of the embodiment of FIG. 4;

FIG. 6 shows a side view of a fourth embodiment of the presentinvention;

FIG. 7 shows a side view of a fifth embodiment of the present invention;

FIG. 8 shows a side view of a sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention is shown in FIG. 1. FIG.1 shows a situation in which the sea is shallow at the point ofmooring--only slightly deeper than the draft of the vessel. The anchorstructure is a circular caisson 10 of size sufficient to resist themooring forces. The caisson 10 is sunk into the sea bed 11. The caissonhas a roof 12 with a circular opening 13. The circular opening is facedwith a wear surface 14, for example made from rubber or timber, thattransmits horizontal forces between the mooring buoy 15 and the caisson10. The mooring buoy 15 contains a variable buoyancy chamber 16 that isused to regulate the buoyancy of the mooring buoy. The mooring buoy 15moors the vessel 20 through the friction developed between the vessel 20and the buoy 15.

When no vessel is moored at the buoy 15, the buoyancy chamber 16 isflooded and the buoy rests on the roof 12 of the caisson 10. When avessel 20 to be moored is directly over the mooring buoy 15, compressedair from a storage tank 23 aboard the buoy is injected into the buoyancychamber 16, and the buoy 15 rises in the water to contact the keel ofthe vessel 20. The vessel 20 is equipped with a pump 21 producingsuction within an area on top of the buoy 15 bordered by the annularseal 17. The vessel can be equipped with any of the devices forproducing suction described in U.S. Pat. Nos. 5,305,703 or 5,477,114,the disclosures of which are incorporated by reference. In consequence,the hydrostatic pressure above buoy 15 is reduced and the buoy 15 ispressed onto the hull of the vessel 20.

The vessel floats in an ice-infested sea with a surface 22. A salientfeature of ice-infested waters is that waves are nearly non-existent. Inconsequence, very little vertical motion between the buoy 15 and thecaisson 10 is caused by waves. Vertical motion from other causes such astide level variation and loading condition can be controlled by theballast water system all ships have; thus the vertical distance betweenthe buoy 15 and the caisson 10 can be kept nearly constant and throughdesign can be kept to a low value, such as a few meters. This may beimportant in order to limit the moment that tends to break the buoy 15away from the hull of the vessel 20.

Although not shown, the roof 12 can be constructed below the sea bed 11such that the top of the buoy 15 is also below the sea bed when novessel is present. At locations with heavy ice this may be desirable toprotect the buoy against direct contact with ice pressure ridges.

FIG. 2 shows an embodiment of the present invention virtually identicalto the embodiment shown in FIG. 1. In this embodiment, the water depthis larger and therefore the caisson 30 protrudes above the sea bed 11and the buoy 15 is attached to the caisson 30 a short distance below thedraft of the vessel 20. In all other respects, this embodiment isidentical to the embodiment shown in FIG. 1.

FIG. 3 shows an embodiment of the invention that is convertible to thetype of mooring described in U.S. Pat. Nos. 5,305,703 and 5,477,114. Inthis embodiment the caisson 40 is not equipped with a fixed roof butwith a lip 41 that prevents a buoyant roof 42 from floating out of thecaisson 40. The buoyant roof 42 has variable flotation tanks 43 that canbe dewatered by the application of pressurized air from a storage tank(not shown) in the roof 42. This system may for example be operated bydivers. In the winter season when the sea surface 22 is ice covered theroof 42 is made buoyant and floats to the position 44. The roof 42 isdesigned with sufficient buoyancy that the moment generated by thehorizontal mooring force cannot tilt the roof. The roof 42 engages aprismatic or cylindrical downward facing element 45 of the mooring buoy15. The mooring forces are then transferred from the buoy 15 to the roof42 through the contact surfaces 46. The force is then transferred fromthe roof 42 to the caisson 40 through the contact surfaces 47. A typicaldiameter of caisson 40 may be 100 m and a typical net buoyancy of theroof 42 in position 44 may be 100 MN.

The roof 42 is in the summer time or open water season ballasted andstored at the floor of caisson 40 in position 48, show in dashed lines.

The buoy 15 is comprised of two parts 51 and 50 separated by a bearing52. The part 50 remains rotationally fixed with respect to the sea bed11 by radial mooring lines 53 anchored to the sea bed at anchors 54. Thepart 51 remains rotationally fixed to the vessel and the bearing 52permits the vessel 20 to weather vane with respect to the sea bed 11.The buoy 15 can be raised or lowered by the variable buoyancy chamber16.

In open water season, the vessel is moored by the buoy 15, which in turnis anchored by the mooring lines 53. This configuration permits largehorizontal and vertical excursions of the buoy 15, thereby securelyanchoring the vessel against the actions of the waves, wind, andcurrent. In the winter or ice season the roof 42 is raised to position44, into engagement with the buoy 15, thereby causing the buoy 15 to beable to withstand the much larger winter time horizontal mooring forces.

FIG. 4 shows in top view another embodiment of the present invention inwhich the vessel 20 is moored to a submerged buoy 15 as in the previousembodiments. The buoy 15 is slidably engaged to an arm 61 which isrotatably connected to the anchor structure 60, which in turn isstructurally connected to the sea bed. This embodiment includes anabove-water rotatable arm 62 which can serve to support fluid connectorsand other cargo transfer equipment permitting the transfer of cargobetween the vessel 20 and the anchor structure 61.

FIG. 5 shows the embodiment of the present invention shown in FIG. 4 ina side view. The buoy 15 can slide vertically with respect to the arm61, which in turn is supported rotatably on the anchor structure 60though the bearing 63. One of the advantages of this embodiment is thatthe anchor structure 60 protrudes above the water through the ice field.In drifting ice the anchor structure 60 would cause a lead or ice breakto be formed in the ice in which the vessel 20 is moored. The leadcauses a reduction in the mooring forces transmitted to the anchorstructure 60 via the buoy 15 and the arm 61. A disadvantage of thisembodiment is that the arm 61 does not automatically align itself withthe lead, therefore when the vessel 20 is approaching it may benecessary to move the arm 61 into alignment with the approaching vesselby applying power to the arm. Such movement may for example be effectedthrough indexing hydraulic cylinders (64) within the bearing 63.

FIG. 6 shows yet another embodiment of the present invention which isparticularly applicable to drilling vessels, but can also be used withproduction and shuttle vessels. The anchor structure is a circularcaisson 70 of size sufficient to resist the mooring forces. The caisson70 is sunk into the sea bed 11. The caisson has a roof 72 with acircular opening 73. The circular opening is faced with a wear surface74, for example made from rubber or timber, that transmits horizontalforces between the mooring buoy 71 and the caisson. 70. The mooring buoyhas a large diameter opening in the center which permits operations tobe performed within the caisson 70 from the deck 76 of the vessel 20.The mooring buoy has a flat annular surface 77 bordered by seals whichare engageable to the mooring area. The mooring buoy 71 containsvariable buoyancy chambers 80 that are used to regulate the buoyancy ofthe mooring buoy 71. The mooring buoy 71 moors the vessel 20 through thefriction developed between the vessel 20 and the buoy 71.

When no vessel is moored at the buoy 71 the buoyancy chambers 80 areflooded and the buoy rests on the roof 72 of the caisson 70. When avessel 20 to be moored is directly over the mooring buoy 71, compressedair from a storage tank aboard the buoy (not shown) is injected into thebuoyancy chambers 80 and the buoy 71 rises in the water to contact thekeel of the vessel 20. The vessel 20 is equipped with a pump 81 takingsuction within an area on top of the buoy 71 bordered by the annularseals 78. In consequence the hydrostatic pressure above buoy 71 isreduced and the buoy 71 is pressed onto the hull of the vessel 20. Thevessel may be equipped with a shaft 82 permitting a drill rig 83 toperform operations on a well head 84 within the caisson 70. Thisarrangement is particularly advantageous because the well head iscompletely protected from the floating ice by the caisson 70 and theroof 72 even when the vessel 20 is not present.

FIG. 7 shows a further embodiment of the present invention, which isparticularly suited for vessels that are either rotationally symmetricalabout a vertical axis or for which the length and the width are nearlythe same. The vessel 91 is shown sitting on top of an anchor structure85 which is structurally fixed to the sea bed 11. The vessel 91 may bebrought over the anchor structure 85 by tug boats (not shown) or bybuilt-in propulsion (not shown). Once the vessel 91 is in position abovethe mooring structure 85 the vessel 91 has its draft increased byballasting. While ballasting, the pump 88 creates suction at the keel ofthe vessel 91, through intake 89. The water pumped by pump 88 isdischarged outside the vessel 91 at discharge 87. When the vessel 91contacts the circumferential sealing element 90 (which sealing element90 may also be on the vessel and contact the mooring structure 85), thewater pressure below the vessel is lowered, as illustrated by theinterior water level 94. As a result, the vessel 91 is forced down ontothe mooring structure 85 with a very large force caused by hydrostaticpressure. Large horizontal mooring forces of the same magnitude may beresisted by the friction between the vessel 91 and the mooring structure85. A particularly advantageous shape of the vessel 91 for iceconditions in the Arctic is illustrated in FIG. 7. The vessel 91 isequipped with a conical surface 86 which promotes breaking of iceimpinging on the vessel 91, and is substantially rotationallysymmetrical about a vertical axis to ensure that ice may be broken nomatter what direction it flows. The vessel 91 may, for example, beequipped with a drilling rig 92 to service a sub-sea well head 93.

FIG. 8 shows another embodiment of the invention substantially similarto the embodiment of FIG. 7. In the embodiment of FIG. 8, the vessel 91is brought into position and ballasted in the same manner as for theembodiment of FIG. 7. The vessel 91 is fitted with a pump 96 that has anintake 98 at the keel and a discharge 97 on the side of the vessel. Whenthe vessel 91 is in position for mooring, the intake is vertically abovean area of the roof of the mooring structure 85 that is bordered byseals 99 that are radially inside and radially outside, respectively,the pump intake 98.

The pressure in the volume defined by the lower end of the vessel 91,the upper end of the mooring structure 85 and the seals 99 is lowered bythe pump 96. Depending on the selection of a suitable pump 96, thepressure may be lowered as far as the vapor pressure of sea water. Ifthe diameters of the seals 99 are 100 m and 50 m respectively, and thedraft of the vessel 91 is 30 m, the resulting attractive force betweenthe vessel 91 and the mooring structure 85 is (π/4)*(100² -50²)*400kN=2300 MN. A friction coefficient of 0.3 between the vessel 91 and theanchor structure 85 will allow a horizontal force of 690 MN to beresisted. This mooring force would typically be sufficient for even thehighest ice or wave forces that the structure could be subjected to inthe ocean, thereby mooring the structure securely.

What is claimed:
 1. An ocean mooring system comprising:a vesselcomprising a hull and an annular mooring area in a bottom of said hull;a buoyant mooring element having an upper part that is engageable withsaid mooring area; an anchoring structure vertically slidably engagedwith a lower part of said mooring element and which is structurallyfixed to a sea bed; means for lowering a hydrostatic pressure in saidmooring area, thereby forcing said mooring element onto said mooringarea; and means for regulating a buoyancy of said mooring element, toraise said mooring element into contact with said hull and to lower saidmooring element away from contact with said hull.
 2. The ocean mooringsystem according to claim 1, wherein:said means for lowering ahydrostatic pressure comprises a water intake in said hull within saidmooring area, said water intake having sufficient flow capacity toremove water leaking past said mooring element into said mooring area.3. The ocean mooring system according to claim 2, wherein:said mooringelement comprises two or more resilient annular members, said resilientannular members making sealing contact at locations in said mooringarea, at least one of said resilient annular members being radiallyoutside said water intake and at least one of said resilient annularmembers being radially inside said water intake.
 4. The ocean mooringsystem according to claim 2, wherein:said mooring element comprises aresilient annular member, said resilient annular member making sealingcontact at a location in said mooring area, said resilient annularmember being radially outside said water intake.
 5. The ocean mooringsystem according to claim 1, wherein:said mooring element comprises oftwo parts separated by a bearing, said bearing allowing said two partsto rotate relative to one another.
 6. The ocean mooring system accordingto claim 1, wherein:said anchoring structure comprises two parts, afirst of said parts being in structural contact with said sea bed and asecond of said parts being movable with respect to said first part, saidsecond part being movable from a first position to a second position,said first position of said second part engaging said mooring element tocreate said slidable engagement, said second position of said secondpart disengaging said second part from said mooring element.
 7. Theocean mooring system according to claim 6, wherein:said mooring elementis anchored to said sea bed with radially deployed anchor lines.
 8. Theocean mooring system according to claim 6, wherein:said second part ismovable by ballasting and deballasting said second part with acompressed gas.
 9. The ocean mooring system according to claim 1,wherein:said means for regulating a buoyancy is a source of compressedgas for ballasting and deballasting said mooring element.
 10. An oceanmooring system comprising:a vessel comprising a hull and an annularmooring area in a bottom of said hull; a buoyant mooring element havingan upper part that is engageable with said mooring area; an anchoringstructure comprising two parts that can rotate about a vertical axisrelative to one another, a first of said parts being structurallyconnected to a sea bed and a second of said parts being rotatablyconnected to said first part and vertically slidably connected to alower part of said mooring element; means for lowering a hydrostaticpressure in said mooring area, thereby forcing said mooring element ontosaid mooring area; and means for regulating a buoyancy of said mooringelement, to raise said mooring element into contact with said hull andto lower said mooring element away from contact with said hull.
 11. Theocean mooring system according to claim 10, wherein:said two parts ofsaid anchoring structure can be rotated mechanically relative to oneanother, thereby to position said mooring element in a proper headingfor mooring said vessel.
 12. An ocean mooring system comprising:a vesselcomprising a hull and an annular mooring area in a bottom of said hull;a buoyant mooring element having an upper part that is engageable withsaid mooring area; an anchoring structure slidably engaged with a lowerpart of said mooring element and which is structurally fixed to a seabed, said anchoring structure comprising two parts, a first of saidparts being in structural contact with said sea bed and a second of saidparts being movable from a first position to a second position, saidfirst position of said second part engaging said mooring element tocreate said slidable engagement, said second position of said secondpart disengaging said second part from said mooring element; means forlowering a hydrostatic pressure in said mooring area, thereby forcingsaid mooring element onto said mooring area; and means for regulating abuoyancy of said mooring element, to raise said mooring element intocontact with said hull and to lower said mooring element away fromcontact with said hull.
 13. The ocean mooring system according to claim12, wherein:said mooring element is anchored to said sea bed withradially deployed anchor lines.
 14. The ocean mooring system accordingto claim 12, wherein:said second part is movable by ballasting anddeballasting said second part with a compressed gas.